U.S. patent application number 15/212787 was filed with the patent office on 2017-01-12 for electrode catalyst for fuel cell, method for producing the same, and polymer electrolyte fuel cell using the same.
This patent application is currently assigned to Toyota Jidosha Kabushiki Kaisha. The applicant listed for this patent is CATALER CORPORATION, TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Akihiro Hori, Yosuke Horiuchi, Mikihiro Kataoka, Tetsuo Nagami, Takahiro Nagata, Sozaburo Ohashi, Tomoaki Terada.
Application Number | 20170012295 15/212787 |
Document ID | / |
Family ID | 42288633 |
Filed Date | 2017-01-12 |
United States Patent
Application |
20170012295 |
Kind Code |
A1 |
Nagami; Tetsuo ; et
al. |
January 12, 2017 |
ELECTRODE CATALYST FOR FUEL CELL, METHOD FOR PRODUCING THE SAME,
AND POLYMER ELECTROLYTE FUEL CELL USING THE SAME
Abstract
A method for producing an electrode catalyst for a fuel cell is
provided. The electrode catalyst includes a carbon support and a
catalyst supported on the carbon support. The catalyst is one of
platinum and a platinum-alloy. The method includes supporting the
catalyst on the carbon support; and treating the carbon support
carrying the catalyst with a nitric acid and cleaning the treated
carbon support, such that an amount of an acid present on the
carbon support becomes in a range from 0.7 mmol to 1.31 mmol of the
acid per gram of the electrode catalyst.
Inventors: |
Nagami; Tetsuo; (Nagoya-shi,
JP) ; Ohashi; Sozaburo; (Nagoya-shi, JP) ;
Kataoka; Mikihiro; (Toyota-shi, JP) ; Terada;
Tomoaki; (Kakegawa-shi, JP) ; Nagata; Takahiro;
(Kakegawa-shi, JP) ; Horiuchi; Yosuke;
(Kakegawa-shi, JP) ; Hori; Akihiro; (Kakegawa-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA
CATALER CORPORATION |
Toyota-shi
Kakegawa-shi |
|
JP
JP |
|
|
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
Toyota-shi
JP
Cataler Corporation
Kakegawa-shi
JP
|
Family ID: |
42288633 |
Appl. No.: |
15/212787 |
Filed: |
July 18, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13379713 |
Dec 21, 2011 |
|
|
|
PCT/IB2010/001019 |
May 4, 2010 |
|
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|
15212787 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/881 20130101;
H01M 4/8842 20130101; H01M 4/921 20130101; Y02E 60/50 20130101;
H01M 4/8663 20130101; H01M 4/926 20130101; H01M 8/1018 20130101;
H01M 4/8878 20130101; H01M 4/8825 20130101; H01M 2008/1095
20130101 |
International
Class: |
H01M 4/92 20060101
H01M004/92; H01M 4/88 20060101 H01M004/88; H01M 8/1018 20060101
H01M008/1018 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2009 |
JP |
2009-147429 |
Claims
1-13. (canceled)
14. A method for producing an electrode catalyst for a fuel cell,
the electrode catalyst including a carbon support; and a catalyst
supported on the carbon support, wherein the catalyst is one of
platinum and a platinum-alloy, the method comprising: supporting
the catalyst on the carbon support; and treating the carbon support
carrying the catalyst with a nitric acid and cleaning the treated
carbon support, such that an amount of an acid present on the
carbon support becomes in a range from 0.7 mmol of the acid per
gram of the electrode catalyst to 1.31 mmol of the acid per gram of
the electrode catalyst.
15. The method according to claim 14, wherein the platinum-alloy
comprises an alloy of platinum and at least one metal selected from
ruthenium, molybdenum, osmium, cobalt, rhodium, iridium, iron
nickel, titanium, tungsten, palladium, rhenium, chromium,
manganese, niobium, and tantalum.
16. The method of claim 14, wherein the carbon support on which the
acid is present and the catalyst is supported is hydrophilic.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a division of application Ser. No. 13/379,713, filed
Dec. 21, 2011, which is a national phase application of
International Application No. PCT/IB2010/001019, filed May 4, 2010,
and claims the priority of Japanese Application No. 2009-147429,
filed Jun. 22, 2009, the contents of all of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to an electrode catalyst for a fuel
cell, a method for producing the electrode catalyst, and a polymer
electrolyte fuel cell using the electrode catalyst, which are
developed so as to improve the performance of a platinum catalyst
or platinum-alloy catalyst.
[0004] 2. Description of the Related Art
[0005] The cell characteristics of polymer electrolyte fuel cells
have remarkably improved for some reasons including the following
reasons: 1) polymer electrolyte membranes having a high ion
conductivity have been developed, and 2) catalyst-carrying carbon
that is coated with a polymer electrolyte of the same type as or a
different type from the polymer electrolyte membrane is used as a
material that forms an electrode catalyst layer, so that
three-dimensional reaction sites are provided in the catalyst
layer. The polymer electrolyte fuel cells, which exhibit the
improved cell performance, can be easily made compact and
lightweight; therefore, it has been expected to put the polymer
electrolyte fuel cells to practical use in mobile vehicles, such as
electric automobiles, or as a power source of a compact
cogeneration system, for example.
[0006] Generally, a gas-diffusible electrode used in a polymer
electrolyte fuel cell consists of a catalyst layer containing the
catalyst-carrying carbon coated with the polymer electrolyte as
described above, and a gas diffusion layer that supplies reaction
gas to the catalyst layer and collects electrons. In the catalyst
layer, porous portions are present in which micropores are formed
among secondary particles or tertiary particles of carbon as a
constituent material of the catalyst layer, and the porous portions
function as diffusion channels for the reaction gas. A catalyst
formed of a noble metal, such as platinum or platinum alloy, which
is stable in the polymer electrolyte is generally used as the
above-mentioned catalyst.
[0007] For example, a catalyst having a noble metal, such as
platinum or platinum alloy, supported on carbon black has been used
as cathode and anode catalysts (electrode catalysts) of polymer
electrolyte fuel cells. Generally, the platinum-carrying carbon
black is prepared by adding sodium hydrogen sulfite to an aqueous
acidic platinum-chloride solution, which is then caused to react
with hydrogen peroxide to form platinum colloid, supporting the
platinum colloid on carbon black, cleaning, and then subjecting the
platinum-carrying carbon black to heat treatment as needed.
[0008] Platinum, which is an expensive noble metal, has been
desired to exhibit sufficient performance even where a small amount
of platinum is carried on a carbon support. Therefore, technologies
for enhancing the catalyst activity with a reduced amount of
platinum used have been studied. For example, an electrode catalyst
for fuel cells as disclosed in Japanese Patent Application
Publication 2002-289208 (JP-A-2002-289208) is composed of an
electrically conductive carbon material, metal particles that are
supported on the conductive carbon material and are less likely be
oxidized than platinum under acidic conditions, and platinum that
covers outer surfaces of the metal particles. The thus formed
electrode catalyst has high durability, and the growth of platinum
particles during operation of the fuel cell is suppressed or
restricted. More specifically, examples of the metal particles
include, for example, alloys comprised of platinum and at least one
metal selected from gold, chromium, iron, nickel, cobalt, titanium,
vanadium, copper, and manganese.
[0009] In order to improve the utilization factor of noble metal
and reduce the amount of noble metal used so as to reduce the cost
of manufacture of electrode catalysts, an electrode catalyst as
disclosed in Japanese Patent Application Publication No. 2005-25947
(JP-A-2005-25947) is prepared by subjecting highly dispersed carbon
having a highly complex structure and a low specific surface area
to a process selected from an activation process, oxidation process
using nitric acid, a process for making the carbon hydrophilic, and
a hydroxyl addition process, so as to activate the surface of the
carbon, and then depositing metal on the carbon.
[0010] However, the platinum catalyst or platinum-alloy catalyst of
the related art is still insufficient in terms of the oxygen
reduction capability, and it has been desired to develop
higher-performance catalysts.
SUMMARY OF THE INVENTION
[0011] The invention provides an electrode catalyst for a fuel
cell, which offers higher performance than the platinum catalyst or
platinum-alloy catalyst of the related art, a method for producing
the electrode catalyst, and a polymer electrolyte fuel cell using
the electrode catalyst.
[0012] A first aspect of the invention is concerned with an
electrode catalyst for a fuel cell, which includes: a carbon
support, a platinum catalyst or a platinum-alloy catalyst supported
on the carbon support, and at least 0.7 mmol of an acid per gram of
the electrode catalyst, which is present on the carbon support.
[0013] To form the electrode catalyst as described above, an acid
treatment is conducted on the carbon support carrying platinum or
platinum alloy, so that 0.7 mmol/g or more of acid per gram of the
catalyst remains on the carbon support. As a result, the catalyst
becomes hydrophilic, and the water-hold property around the
catalyst is improved, resulting in a reduction in the resistance to
proton shift in the catalyst layer, and improved power generation
performance at low humidity.
[0014] In the electrode catalyst according to the first aspect of
the invention, the platinum-alloy catalyst may consist of an alloy
of platinum and at least one metal selected from ruthenium,
molybdenum, osmium, cobalt, rhodium, iridium, iron, nickel,
titanium, tungsten, palladium, rhenium, chromium, manganese,
niobium, and tantalum.
[0015] In the electrode catalyst according to the first aspect of
the invention, the carbon support on which the acid is present and
the platinum catalyst or the platinum-alloy catalyst is supported
may be hydrophilic.
[0016] In the electrode catalyst according to first aspect of the
invention, the acid may be present on the carbon support in an
amount equal to or greater than 1.0 mmol per gram of the electrode
catalyst, or may be present on the carbon support in an amount
equal to or greater than 1.31 mmol per gram of the electrode
catalyst.
[0017] A second aspect of the invention is concerned with a method
for producing an electrode catalyst for a fuel cell, which
comprises a carbon support and a platinum catalyst or a
platinum-alloy catalyst supported on the carbon support. The method
includes the steps of: supporting the platinum catalyst or the
platinum-alloy catalyst on the carbon support, and treating the
carbon support carrying the platinum catalyst or the platinum-alloy
catalyst with an acid, so that at least 0.7 mmol of the acid per
gram of the electrode catalyst is present on the carbon support
treated with the acid.
[0018] In the method according to the second aspect of the
invention, after the carbon support carrying the platinum catalyst
or the platinum-alloy catalyst is treated with the acid, the carbon
support carrying the platinum catalyst or the platinum-alloy
catalyst and treated with the acid may be cleaned.
[0019] In the method according to the second aspect of the
invention, the platinum-alloy catalyst may consist of an alloy of
platinum and at least one metal selected from ruthenium,
molybdenum, osmium, cobalt, rhodium, iridium, iron, nickel,
titanium, tungsten, palladium, rhenium, chromium, manganese,
niobium, and tantalum.
[0020] In the method according to the second aspect of the
invention, the carbon support on which the acid is present and the
platinum catalyst or the platinum-alloy catalyst is supported may
be hydrophilic.
[0021] In the method according to the second aspect of the
invention, the acid may be present on the carbon support in an
amount equal to or greater than 1.0 mmol per gram of the electrode
catalyst, or may be present on the carbon support in an amount
equal to or greater than 1.31 mmol per gram of the electrode
catalyst.
[0022] A third aspect of the invention is concerned with a polymer
electrolyte fuel cell including the electrode catalyst according to
the first aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The foregoing and further objects, features and advantages
of the invention will become apparent from the following
description of example embodiments with reference to the
accompanying drawings, wherein:
[0024] FIG. 1 is a graph showing the relationship between the
catalyst acid amount and the low-humidity efficiency point
performance (at 0.2 A/cm.sup.2) with regard to Examples 1, 2 and
Comparative Examples 1-3;
[0025] FIG. 2 is a graph showing the relationship between the
catalyst acid amount and the low-humidity output point performance
(at 1.02 A/cm.sup.2) with regard to Examples 1, 2 and Comparative
Examples 1-3;
[0026] FIG. 3 is a graph showing the relationship between the
catalyst acid amount and the low-humidity efficiency point
performance (at 0.2 A/cm.sup.2) with regard to Examples 3-7 and
Comparative Examples 4-14; and
[0027] FIG. 4 is a graph showing the relationship between the
catalyst acid amount and the low-humidity output point performance
(at 1.02 A/cm.sup.2) with regard to Examples 3-7 and Comparative
Examples 4-14.
DETAILED DESCRIPTION OF EMBODIMENTS
[0028] The inventors reached the present invention by subjecting
carbon carrying a platinum catalyst or platinum-alloy catalyst to a
particular treatment so as to bring it into a particular carbon
state.
[0029] In the following, some examples of this invention and
comparative examples will be described in detail. A process for
producing a single cell used for evaluation, a method of evaluating
the performance of a catalyst of each example, and a method of
determining the amount of acid in the catalyst will be described
below.
[0030] The process for producing a single cell used for evaluation
will be described. A single cell for use in a polymer electrolyte
fuel cell was formed in the following manner, using a catalyst
powder obtained in each example or comparative example. The
catalyst powder was dispersed in an organic solvent, and the
resulting dispersion liquid was applied by coating to a Teflon
sheet to form catalyst layers (i.e., electrodes). The amount of Pt
catalyst per 1 cm.sup.2 of electrode area was 0.4 mg. The
electrodes formed from the catalyst powder were attached to each
other via a polymer electrolyte membrane by hot press, to provide a
membrane-electrode assembly, and diffusion layers were mounted on
the opposite sides of the membrane-electrode assembly, to form a
single-cell electrode.
[0031] The method of evaluating the catalyst performance will be
described. To evaluate the catalyst performance, the initial
voltage measurement was conducted in the following manner. The
temperature of the single cell was set to 80.degree. C., and
moisturized air that passed a bubbler heated to 60.degree. C. was
supplied to the cathode-side electrode at a rate of 2.0 L/min,
while moisturized hydrogen that passed a bubbler heated to
60.degree. C. was supplied to the anode-side electrode at a rate of
0.5 L/min. In this condition, current voltage characteristics were
measured. Comparisons of the performance among the catalysts of the
respective examples were made through measurements of voltage
values at current densities of 0.2 A/cm.sup.2 and 1.0
A/cm.sup.2.
[0032] The method of determining the acid amount in the catalyst
will be described. After 0.5 g of catalyst was added to 20 ml of
0.1N sodium hydroxide, which was then ultrasonically stirred for 20
min., the resulting liquid was subjected to filtration. Then, 0.05
ml of Methyl Orange as an indicator was added to 5 ml of filtrate
while it was being stirred, and titration was conducted with 0.05N
hydrochloric acid.
[0033] Example 1 will be described. Initially, 4.2 g of Ketjen EC
(manufactured by Ketjen Black International Company, JAPAN), which
is commercially available, and 5.0 g of platinum were added to and
dispersed in 0.5 L of pure water. About 100 mL of 0.1N ammonia was
then added to the resulting liquid to make PH equal to about 10, so
that a hydroxide was formed and deposited on carbon. The resulting
dispersion liquid was subjected to filtration, and the obtained
powder was dried at 100.degree. C. in a vacuum for 10 hours. Then,
the powder was held at 400.degree. C. for 2 hours in hydrogen gas
so as to be reduced, and then held at 1000.degree. C. for 10 hours
in nitrogen gas so as to provide a catalyst powder. The obtained
catalyst was thrown into 1 L of 0.5N nitric acid, heated to
80.degree. C., and was stirred for 30 min. Then, the catalyst was
isolated by filtration, and was dried in a blowing drier at
80.degree. C. for 15 hours or longer, to provide a catalyst powder.
The acid amount in the catalyst was measured, and the result of the
measurement was 1.020 mmol/g-cat.
[0034] Example 2 will be described. A catalyst powder as Example 2
was obtained by preparing a catalyst in the same manner as in
Example 1, except that, after the catalyst powder was treated with
the acid, it was dried in a vacuum drier at 60.degree. C. for 15
hours or longer. The amount of acid in the catalyst was 1.156
mmol/g-cat.
[0035] Comparative Example 1 will be described. A catalyst powder
as Comparative Example 1 was obtained by preparing a catalyst in
the same manner as in Example 1, except that the acid treatment
(i.e., a process of treating the catalyst powder with an acid) was
not conducted. The amount of acid in the catalyst was 0.52
mmol/g-cat.
[0036] Comparative Example 2 will be described. A catalyst powder
as Comparative Example 2 was obtained by preparing a catalyst in
the same manner as in Example 1, except that, after the acid
treatment was conducted, the catalyst was filtered and cleaned with
1 L of pure water, and the filtration and cleaning were repeated
until the conductivity of drainage or waste liquid became equal to
or lower than 20 .mu.S/cm. The amount of acid in the catalyst was
0.628 mmol/g-cat.
[0037] Comparative Example 3 will be described. A catalyst power as
Comparative Example 3 was obtained by preparing a catalyst in the
same manner as in Example 1, except that, after the acid treatment
was conducted, the catalyst was filtered and cleaned with 1 L of
pure water only once. The amount of acid in the catalyst was 0.996
mmol/g-cat.
[0038] FIG. 1 shows the relationship between the catalyst acid
amount and the low-humidity efficiency point performance (at 0.2
A/cm.sup.2) with regard to Examples 1, 2 and Comparative Examples
1-3 as described above. FIG. 2 shows the relationship between the
catalyst acid amount and the low-humidity output point performance
(at 1.02 A/cm.sup.2) with regard to Examples 1, 2 and Comparative
Examples 1-3.
[0039] As is understood from FIG. 1 and FIG. 2, Examples of the
invention showed high voltage values at both of the current
densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2, since the catalysts
of these Examples had an acid that can be hydrophilic. On the other
hand, Comparative Examples showed low voltage values at both of the
current densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2. It is
concluded from these results that the catalyst becomes hydrophilic
when it contains an acid that can be hydrophilic, and the
water-hold property around the catalyst is improved, resulting in a
reduction in the resistance to proton shift in the catalyst
layer.
[0040] Comparative Example 4 will be described. Initially, 4.71 g
of a commercially available carbon powder having a high specific
surface area was added to and dispersed in 0.5 L of pure water, to
provide a dispersion liquid. A hexahydroxo platinum nitric acid
solution containing 4.71 g of platinum and an aqueous solution of
cobalt nitrate containing 0.592 g of cobalt were dropped in this
order into the dispersion liquid, to be sufficiently brought into
contact with carbon. Then, about 5 mL of 0.01N ammonia was added to
the resulting liquid to make PH equal to about 9, so that a
hydroxide was formed and deposited on the carbon. The resulting
dispersion liquid was repeatedly filtered and cleaned until the
conductivity of filtration drainage became equal to or lower than
50 .mu.S/cm, and the obtained powder was dried in a vacuum at
100.degree. C. for 10 hours. Then, after the dried powder was held
in hydrogen gas at 500.degree. C. for 2 hours so as to be reduced,
it was held in nitrogen gas at 700.degree. C. for 0.5 hour and held
in the same gas at 600.degree. C. for 6 hours, to provide an alloy
of platinum and cobalt.
[0041] Furthermore, the catalyst powder was thrown into 0.5 L of
0.5N nitric acid, heated to 80.degree. C., and was stirred for 30
min, so that cobalt that had not been alloyed was removed by acid
cleaning. Then, the catalyst was filtered and cleaned with 1 L of
pure water, and the filtration and cleaning were repeated until the
conductivity of the cleaning drainage became equal to or lower than
20 .mu.S/cm. Then, the catalyst was isolated by filtration, and was
dried in a vacuum drier at 100.degree. C. for 12 hours or longer,
to provide a catalyst powder as Comparative Example 4.
[0042] Comparative Example 5 will be described. A catalyst powder
as Comparative Example 5 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 700.degree. C. for
0.5 hour and held in the same gas at 600.degree. C. for 12 hours,
to provide an alloy of platinum and cobalt.
[0043] Comparative Example 6 will be described. A catalyst powder
as Comparative Example 6 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 700.degree. C. for
0.5 hour and held in the same gas at 600.degree. C. for 18 hours,
to provide an alloy of platinum and cobalt.
[0044] Comparative Example 7 will be described. A catalyst powder
as Comparative Example 7 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 700.degree. C. for
6.5 hours, to provide an alloy of platinum and cobalt.
[0045] Comparative Example 8 will be described. A catalyst powder
as Comparative Example 8 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 800.degree. C. for
6.5 hours, to provide an alloy of platinum and cobalt.
[0046] Comparative Example 9 will be described. A catalyst powder
as Comparative Example 9 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 700.degree. C. for
0.5 hour, to provide an alloy of platinum and cobalt.
[0047] Comparative Example 10 will be described. A catalyst powder
as Comparative Example 10 was obtained in the same manner as in
Comparative Example 4, except for the following steps. In
Comparative Example 10, after the reduction process, the catalyst
powder was held in nitrogen gas at 700.degree. C. for 0.5 hour, to
provide an alloy of platinum and cobalt. Furthermore, the catalyst
powder was thrown into 0.5 L of 0.5N nitric acid, heated to
80.degree. C., and was stirred for 30 min, so that cobalt that had
not been alloyed was removed by acid cleaning. Then, in Comparative
Example 10, the catalyst was isolated by filtration but not cleaned
with pure water, and was dried in a vacuum drier at 100.degree. C.
for 12 hours or longer.
[0048] Comparative Example 11 will be described. A catalyst powder
as Comparative Example 11 was obtained in the same manner as in
Comparative Example 4, except for the following steps. In
Comparative Example 11, after the reduction process, the catalyst
powder was held in nitrogen gas at 700.degree. C. for 0.5 hour, to
provide an alloy of platinum and cobalt. Furthermore, the catalyst
powder was thrown into 0.5 L of 0.5N nitric acid, heated to
80.degree. C., and was stirred for 30 min, so that cobalt that had
not been alloyed was removed by acid cleaning. Then, in Comparative
Example 11, the catalyst was isolated by filtration but not cleaned
with pure water, and was dried in a blowing drier at 80.degree. C.
for 12 hours or longer.
[0049] Example 3 will be described. A catalyst powder as Example 3
was obtained in the same manner as in Comparative Example 4, except
that, after the alloying process, the catalyst powder was thrown
into 0.5 L of 2N nitric acid, heated to 80.degree. C., and was
stirred for 30 min, so that cobalt that had not been alloyed was
removed by acid cleaning. Then, in Example 3, the catalyst was
isolated by filtration but not cleaned with pure water, and was
dried in a vacuum drier at 100.degree. C. for 12 hours or
longer.
[0050] Example 4 will be described. A catalyst powder as Example 4
was obtained in the same manner as in Comparative Example 4, except
for the following steps. In Example 4, after the alloying process,
the catalyst powder was thrown into 0.5 L of 2N nitric acid, heated
to 80.degree. C., and was stirred for 30 min, so that cobalt that
had not been alloyed was removed by acid cleaning. Then, the
catalyst was filtered and cleaned with 1 L of pure water, and the
filtration and cleaning were repeatedly conducted until the
conductivity of the cleaning drainage became equal to or lower than
20 .mu.S/cm. The catalyst was isolated by filtration, and was
further thrown into 0.5 L of 0.5N nitric acid and stirred for 30
min at room temperature. Thereafter, the catalyst was isolated by
filtration but not cleaned with pure water, and was dried in a
vacuum drier at 100.degree. C. for 12 hours or longer.
[0051] Example 5 will be described. A catalyst powder as Example 5
was obtained in the same manner as in Comparative Example 4, except
for the following steps. In Example 5, after the alloying process,
the catalyst powder was thrown into 0.5 L of 2N nitric acid, heated
to 80.degree. C., and was stirred for 30 min, so that cobalt that
had not been alloyed was removed by acid cleaning. Then, the
catalyst was filtered and cleaned with 1 L of pure water, and the
filtration and cleaning were repeatedly conducted until the
conductivity of the cleaning drainage became equal to or lower than
20 .mu.S/cm. The catalyst was isolated by filtration, and was
further thrown into 0.5 L of 0.5N nitric acid and stirred for 30
min at room temperature. Thereafter, the catalyst was isolated by
filtration but not cleaned with pure water, and was dried in a
blowing drier at 80.degree. C. for 12 hours or longer.
[0052] Example 6 will be described. A catalyst powder as Example 6
was obtained in the same manner as in Comparative Example 4, except
that, after the reduction process, the catalyst powder was held in
nitrogen gas at 700.degree. C. for 0.5 hour, to provide an alloy of
platinum and cobalt, and the catalyst powder was thrown into 0.5 L
of 0.5N nitric acid, heated to 80.degree. C., and was stirred for
48 hours, so that cobalt that had not been alloyed was removed by
acid cleaning.
[0053] Comparative Example 12 will be described. A catalyst powder
as Comparative Example 12 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 800.degree. C. for
0.5 hour, to provide an alloy of platinum and cobalt.
[0054] Comparative Example 13 will be described. A catalyst powder
as Comparative Example 13 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 700.degree. C. for
0.05 hour, to provide an alloy of platinum and cobalt.
[0055] Example 7 will be described. A catalyst powder as Example 7
was obtained in the same manner as in Comparative Example 4, except
that, after the reduction process, the catalyst powder was held in
nitrogen gas at 800.degree. C. for 0.5 hour, to provide an alloy of
platinum and cobalt, and that the catalyst powder was thrown into
0.5 L of 0.5N nitric acid, heated to 80.degree. C., and was stirred
for 48 hours, so that cobalt that had not been alloyed was removed
by acid cleaning.
[0056] Comparative Example 14 will be described. A catalyst powder
as Comparative Example 14 was obtained in the same manner as in
Comparative Example 4, except that, after the reduction process,
the catalyst powder was held in nitrogen gas at 800.degree. C. for
0.5 hour, to provide an alloy of platinum and cobalt, and the
catalyst powder was thrown into 0.5 L of 0.05N nitric acid, heated
to 80.degree. C., and was stirred for 48 hours, so that cobalt that
had not been alloyed was removed by acid cleaning.
[0057] TABLE 1 below shows, in list form, the remaining acid amount
and the power generation performance with regard to Examples 3-7
and Comparative Examples 4-14. FIG. 3 shows the relationship
between the amount of acid in the catalyst and the low-humidity
efficiency point performance (at 0.2 A/cm.sup.2), with regard to
Examples 3-7 and Comparative Examples 4-14. FIG. 4 shows the
relationship between the amount of acid in the catalyst and the
low-humidity output point performance (at 1.02 A/cm.sup.2), with
regard to Examples 3-7 and Comparative Examples 4-14.
TABLE-US-00001 TABLE 1 Initial Cell Performance Low Humidity
Catalyst Powder (Both Electrodes RH = 40) Acid Amount by
Efficiency-point Output-point Back Titration Voltage at Voltage at
mmol/g 0.2 A/cm.sup.2(V) 1.0 A/cm.sup.2(V) Com. Ex. 4 0.40 0.708
0.476 Com. Ex. 5 0.56 0.716 0.402 Com. Ex. 6 0.52 0.713 0.408 Com.
Ex. 7 0.44 0.724 0.421 Com. Ex. 8 0.44 0.690 0.378 Com. Ex. 9 0.47
0.729 0.436 Com. Ex. 10 0.56 0.735 0.444 Com. Ex. 11 0.63 0.761
0.508 Example 3 1.01 0.765 0.514 Example 4 1.05 0.764 0.528 Example
5 1.31 0.758 0.525 Example 6 0.80 0.772 0.548 Com. Ex. 12 0.43
0.730 0.457 Com. Ex. 13 0.46 0.729 0.438 Example 7 0.73 0.763 0.532
Com. Ex. 14 0.55 0.760 0.521
[0058] As is understood from FIG. 3 and FIG. 4, the platinum-alloy
catalysts (Examples 3-7) according to the invention showed high
voltage values at both of the current densities, 0.2 A/cm.sup.2 and
1.02 A/cm.sup.2, since the catalysts of these examples had an acid
that can be hydrophilic. On the other hand, Comparative Examples
showed low voltage values at both of the current densities, 0.2
A/cm.sup.2 and 1.02 A/cm.sup.2. It is concluded from these results
that the catalyst becomes hydrophilic when it contains an acid that
can be hydrophilic, and the water-hold property around the catalyst
is improved, resulting in a reduction in the resistance to proton
shift in the catalyst layer.
[0059] In practicing the present invention, carbon carrying a known
platinum catalyst or known platinum-alloy catalyst may be used.
Also, in practicing the present invention, various types of acids
may be used in an acid treatment performed on the platinum or
platinum-alloy carrying carbon, and nitric acid may be preferably
used.
[0060] While some embodiments of the invention have been
illustrated above, it is to be understood that the invention is not
limited to details of the illustrated embodiments, but may be
embodied with various changes, modifications or improvements, which
may occur to those skilled in the art, without departing from the
scope of the invention.
[0061] The electrode catalyst for fuel cells according to the
invention has a higher activity than the platinum catalyst or
platinum-alloy catalyst of the related art, thus making it possible
to reduce the amount of expensive platinum used in the
catalyst.
* * * * *